Non vibrating capacitance probe for wear monitoring

ABSTRACT

A non-vibrating capacitance probe for use as a non-contact sensor for tribological wear on a component. The device detects surface charge through temporal variation in the work function of a material. A reference electrode senses changing contact potential difference over the component surface, owing to compositional variation on the surface. Temporal variation in the contact potential difference induces a current through an electrical connection. This current is amplified and converted to a voltage signal by an electronic circuit with an operational amplifier.

STATEMENT OF RELATED APPLICATIONS

This application is based and claims priority on United States ofAmerica provisional patent application Ser. No. 60/030,814, filed onNov. 14, 1996.

STATEMENT OF GOVERNMENT INTEREST

Part of the work for this invention was funded by the United States ofAmerica Office of Naval Research under contracts numbersN00014-95-1-0903 and N00014-94-1-1074. The government of the UnitedStates of America has certain rights to this invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to non-contact sensors formonitoring surface variations of a component part, and more specificallyrelates to a non-vibrating capacitance probe which uses a variablecapacitor to measure the contact potential difference between twosurfaces, generally on the same component part, and thereby recognizessurface variations such as wear of an object subjected to, for example,a sliding contact.

2. Technical Field

Mechanical systems such as heat combustion engines have components thatare dynamically in contact with another body. These components aresubjected to cyclic motions that can involve impact loading, shearstraining, plastic deformation, frictional heating, and fatigue ofsub-surface regions. A combination of these mechanisms often leads tosurface damage that impairs the performance of the component. Inaddition, the chemical interaction between the component surface andsurrounding fluids also can accelerate surface degradation. Suchproblems, if unattended, can result in catastrophic malfunction of themachine and even compromise operational safety. In this regard, it isdesirable to monitor the surface condition of a critical tribocomponent.The design of sensors to monitor the surface condition of thetribocomponents and the operation of machinery depends largely on thenature of tribological application.

A surface-monitoring method that exploits the spatial variation in thework function of a material is presented herein. The work functionrefers to an energy barrier to prevent the escape of electrons from thesurface of the material. The work function is governed by thephysio-chemical nature of the surface and also depends on theenvironmental conditions. From a tribological standpoint, the workfunction is a useful parameter for evaluating mechanical deformationfeatures such as dislocation pile-ups and residual stresses. Forexample, it has been demonstrated that a metal subjected to differentdegrees of compressive stress exhibits a variation in the work function.Craig, P. P. and Radeka, V., “Stress Dependence of Contact Potential:The ac Kelvin Method,” Rev. Sci. Instrum., Vol. 41, pp. 258-264, 1969.The present invention is a non-vibrating capacitance probe as modifiedfrom that of the Kelvin-Zisman method, Zisman, W. A., “A New Method ofMeasuring Contact Potential Differences in Metals,” Sci. Instrum., Vol.3, pp. 367-370, 1932, that uses a variable capacitor to measure thecontact potential difference (CPD) between two surfaces.

SUMMARY OF THE INVENTION

Briefly described, in a preferred form, the present invention monitorsthe surface variations, such as surface wear, of a component. Thesurface wear is measured by the spatial variation in the work functionof the component. The work function refers to an energy barrier toprevent the escape of electrons from the surface of the component. Theinvention detects the surface charge of the surface of the componentthrough temporal variation in the work function of the component.

The present invention generally comprises the novel combination of ameans for supporting the component and a non-vibrating capacitanceprobe, and the use of the non-vibrating capacitance probe in thiscombination to carry out the wear monitoring function of this invention.The component and non-vibrating probe are located in close proximity toeach other. The relative motion between the component and thenon-vibrating probe, the distance between them, and the contactpotential difference between them, all are monitored. The work functionof the component is found by monitoring the current induced by contactpotential difference in the non-vibrating probe and relating it to theknown work function of the electrode in the probe.

The present invention is directed to a non-vibrating capacitance probewhich may be used as a non-contact sensor for tribological wear.Specifically, the present invention is a device which detects surfacecharge through temporal variation in the work function of a material. Anartificial spatial variation in the work function is imposed on a shaftsurface by coating a segment along the shaft circumference with a metalpaint wherein the paint is compositionally different than the shaftsurface. As the shaft rotates, the reference electrode senses changingcontact potential difference with the shaft surface, owing tocompositional variation. Temporal variation in the contact potentialdifference induces a current through an electrical connection. Thiscurrent is amplified and converted to a voltage signal by an electroniccircuit with an operational amplifier. The magnitude of the signaldecreases asymptotically with the electrode-shaft distance and increaseslinearly with the rotational frequency.

In one embodiment of the apparatus, the component to be monitored forsurface variations either is a cylindrical shaft composed of thematerial to be monitored, or wear-tested, or is a cylindrical shaftcoated with the material to be monitored, or wear-tested. The componentis supported by roller bearings on both ends of the shaft, allowingrotation of the shaft along its axis. The shaft is rotated by a motorand the rotational speed of the shaft is monitored. A non-vibratingcapacitance probe is mounted on an xyz-positioning system, and a monitordetects the spacing between the shaft surface and probe. A monitoringdevice interprets the current induced in the non-vibrating capacitanceprobe as a difference in work function between the component and theknown work fimction of the reference electrode in the probe. The processof measuring the work function of the component comprises the creationof relative rotational motion between the component and thenon-vibrating capacitance probe. The relative motion of the componentand probe, and the distance between the component and probe also aremonitored.

One application of the non-vibrating capacitance probe is for detectingsurface wear of an object subjected to sliding contact. One technique isto apply a thin coating of a material on the sliding body that iscompositionally different from the substrate. Partial removal of thiscoating due to sliding contact creates sites where the substratematerial is exposed. Formulation of these sites create lateralcompositional variation, thus, heterogeneity in the work function of thewear surface. This yields an induced-current pattern that is unique fromthat of the unworn surface coating.

Accordingly, it is a primary object of the present invention to providean apparatus comprising a non-vibrating capacitance probe which can beused as a non-contact sensor for tribological wear.

It is another object of the present invention to provide an apparatuscomprising a non-vibrating capacitance probe which can be miniaturizedand installed in systems that have moving parts.

These and other objects, advantages, and features of the presentinvention will become apparent to those skilled in the art upon readingthe following specification in conjunction with the accompanying drawingfigures, in which like reference numerals designate like partsthroughout the several views.

DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 is a schematic of the Kelvin-Zisman method (prior art).

FIG. 2 is a graph of CPD variation measured by the present inventionbetween the reference electrode and a rotating cylindrical surfacecomposed of materials A and B.

FIGS. 3(a) and 3(b) show the theoretical variation of dV/dt with timefor different values of x.

FIG. 4 shows the theoretical maximum dV/dt plotted as a function offrequency.

FIG. 5 shows the experimental set-up for a preferred embodiment of thepresent invention.

FIG. 6 is a circuit diagram of the non-vibrating capacitance probe,according to one form of the present invention.

FIGS. 7(a) and 7(b) show experimental samples of probe output signal fordifferent values of x.

FIG. 8 shows the magnitude of maximum output plotted as a function ofprobe-sample distance.

FIG. 9 shows a linearized plot of maim output as a function of probedistance.

FIG. 10 shows the magnitude of maximum output plotted as a function ofrotational frequency.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Theoretical

The theoretical background detailed below provides a description of theKelvin-Zisman method to monitor a surface probe, and demonstrates theoperation of the probe on a rotating shaft.

Kelvin-Zisman Probe

Referring to FIG. 1, the Kelvin-Zisman technique is accomplished bycreating a parallel plate dynamic or vibrating capacitor 10 by vibratingone plate, the reference electrode 12, relative to a second plate, thesample surface 14 of interest. The surface 14 corresponds to thecomponent subject to wear or to having other surface variations. Thevibration induces a current flow, i, which can be described in terms ofthe geometry of the capacitor 10 and difference in work function betweenthe reference electrode 12 and surface 14. If the work function of thereference electrode 12, φ_(ref), is known, then the changes in the workfunction of the surface 14, φ_(desired), can be related to whateverexperimental conditions are chosen. The general equation for theinducted current isi=V(dC/dr)+C(dV/dr)  (1)where V, the CPD voltage, is defined byV=(φ_(ref)−φ_(desired))/e  (2)and C, the capacitance, is expressed asC=ε_(r)ε₀A/d  (3)

where e is the charge of an electron ε_(r) is the relative dielectricconstant, ε₀ is the permitivity in free space, A is the area of thereference electrode, and d is the spacing between the surfaces.

A typical experimental condition involves a reference electrode thatdoes not detect a varying φ_(desired), thus, the term dV/dt in equation1 is assumed to be zero. In most CPD-measurement studies, such acondition is implemented by having the vibrating reference electrodefixed in position on a particular site of the sample surface. Theinduced current is contributed solely by the change in the capacitanceowing to the sinusoidal variation in d expressed asd=d₀+d₁sinωt  (4)

where d₀ is the mean spacing, d₁ is the amplitude, ω is the angularfrequency, and t is the time. Substituting equation 4 into equation 3and 1 yieldsi=−Vε_(r)ε₀Ad₁ cos ωt/(d₀+d₁ sin ωt)²  (5)

The Kelvin-Zisman technique to measure V is to provide a compensatingvoltage, V_(C), to the capacitor 10, shown in FIG. 1, so that i=0. Thedc voltage could be applied either externally or through a feedbackcircuit via a phase sensitive detector.

Inventive Embodiment

In preferred form, the present invention comprises a non-vibratingcapacitance probe for surface wear monitoring. The probe of the presentinvention forms a capacitor 10 between a reference electrode 12 and asurface 14 of interest, as described by the Kelvin-Zisman techniqueabove. However, the spacing between the two surfaces, the electrode 12being the first surface and the surface 14 being the second surface, inthe present invention is fixed. Instead of the variable capacitance, thecurrent is induced by the temporal change in CPD. Therefore, inreference to equation 1, the formulation for the induced current issimplified toi=C(dV/dt)  (6)

Varying the CPD with time can be achieved by imposing a lateraldisplacement between the reference electrode 12 and the sample surface14 with a heterogeneous work function. A combination of equation 6 withequation 3, which yieldsi=ε_(r)ε₀A(dV/dt)/d,  (7)suggests that the magnitude of the induced current decreasesasymptotically with the capacitor spacing, and increases with the areaof the reference electrode and the rate of CPD change.

One embodiment of the present invention is the scanning of a cylinder 30having a cylindrical surface 20 rotating along its longitudinal axis 22,as shown in FIG. 2. Using the geometry depicted in FIG. 2, along thecircumference of the cylinder 30, part of the surface 20 consists ofmaterial A, and the rest of the surface 20 consists of material B; eachmaterial having a unique work function.

As the cylinder 30 rotates at a constant speed, the reference electrode40 senses a contact potential difference with material A, CPD_(EA), andanother potential with material B, CPD_(EB). Also assume the CPD_(EB) iszero. The variation in the CPD with time can be described by arectangular wave function V(t) with an amplitude CPD_(EA), as shown inFIG. 2. The Fourier series of the function isV(t)=V′x+V′/Π{Σ[(sin(2Πnx)/n)cos(wΠfnt)]+[(1−cos(wΠnx)/n)sin(2Πfnt)]}  (8)

wherein V′=CPD_(EA)−CPD_(EB), in volts, f is the fundamental frequencywhich is equivalent to the rotational frequency, x is the ratio of thearc length of A to the circumference of the cylinder, and n=1, 2, 3, . .. ∞. The derivative of this function is defined bydV/dt=−2V′ƒ{Σ[(sin(eΠnx)/n)sin(2Πfnt)]+[(1−cos(2Πnx)/n)cos(2Πfnt)]}  (9)

For CPD_(EB)≠0, the derivative of V(t) is still identical to equation 9where the dc component is eliminated.

FIG. 3 shows plots of equation 9 for x values of 0.013 and 0.3. Forthese calculations, V′=1, f=15 Hz, and n=1 to 10. Each cycle of the waveconsists of two major peaks, one with positive, maximum, value, and theother with negative, minimum, value. These peaks define the boundariesof material A where there are sharp changes in the CPD. The gap betweenthe peaks widens as the length fraction of A increases.

Equation 9 indicates that the magnitude of the peak depends on thefundamental frequency. This is illustrated in FIG. 4 that reveals alinear increase in maximum dV/dt from 10 to 20 Hz. For this plot, x isfixed at 0.013 and V′ and n are the same as for FIG. 3.

It should be noted that waves with smaller amplitude separate the majorpeaks as shown in FIG. 3. There should be a straight line (dV/dt=0)instead because of the absence of CPD variation between materialboundaries. The appearance of minor waves between the large peaks isattributed to the limited number of harmonics included in thecalculation. With the higher number of harmonics, the amplitude of thesewaves approaches zero.

Another embodiment of the present invention, as shown in FIG. 5,comprises an aluminum shaft 100 rotated by a stepper motor 110. Bothends of the shaft 100 are supported by roller bearings 112, 114. One endof the shaft 100 is connected to the motor spindle 116 with a coupling118. Interfaced with the motor 110 is a control box 120 for regulatingthe rotational speed of the shaft 100. The entire mechanical assembly ismounted on a vibration-isolation table 130. The rotational frequency ofthe shaft 100 is monitored by a tachometer 140. In the described sets ofexperiments, the rotational frequency was set at 10, 15, 20, and 25 Hz,corresponding to 600, 900, 1200, and 1500 rpm. The experimental shaft100 was about 432 mm in length and about 50.8 mm in diameter.

A non-vibrating capacitance probe 150 is mounted on an xyz positioningsystem 160 which is mechanically isolated from the above set-up. Steppermotors, not shown, control the lateral motion of the probe 150 along thelongitudinal axis of the shaft 100 and the vertical position. The probe150 is positioned such that a reference electrode 152 in the probe 150is perpendicular to the shaft 100 surface. A separate positioning stagewith a translational resolution of 0.01 mm is used to manually adjustthe spacing between the shaft 100 and the reference electrode 152.Spacings ranging from 01. to 1.25 were used in experimentation.

Artificial variation in the work function was imposed on the sampleshaft 100 surface by coating a segment along the shaft 100 circumferencewith a colloidal silver paint. Most of the tests were conducted for asilver strip 170 with an arc length x that was 1.3/100, or 0.013, of thecircumferencial length of the shaft 100. One test was performed for aseparate coating with a length x fraction of 0.3. The coating stripswere approximately 14-μm thick and 5-mm wide for this experimentation.

The reference electrode 152 of the probe 150 was made of lead wire witha cross-sectional area of approximately 0.446 mm². Electrical connectionbetween the sample shaft 100 and the common ground of the probe's 150electronic circuit was maintained through a brush in contact with theshaft 100. The current induced by the time-varying CPD between theelectrode 152 and rotating shaft 100 surface was converted to a voltageoutput, as shown in FIG. 6, via a high ohmic circuit with a gain factorof 3.9×10⁹ V/amp. The operational amplifier in the circuit received a dcpower of ±9 V. The voltage output of the amplifier was recorded by adata acquisition system 180 at a rate of 10 kHz.

FIG. 7a shows an example of signal output for the silver strip 170 witha length fraction of 0.013. The signal exhibits a series of large waves,separated by fluctuations with smaller amplitudes. This pattern isidentical to that of the theoretical signal which is calculated for asimilar length fraction, shown in FIG. 3a. The time interval between thelarge waves corresponds to the rotational frequency of the shaft 100.The interval between the maximum and minimum peaks of each wave packetrepresents the traverse of the probe 150 along the arc length of thesilver strip 170. As per FIG. 2, upon entry into the silver strip 170,the reference electrode 152 senses an abrupt shift in the contactpotential difference from aluminum to silver. At this point, the rate ofchange in CPD, ie., dV/dt, is maximum (equation 7). As the referenceelectrode 152 moves from silver to aluminum, it senses another sharpchange in CPD but with a dV/dt of reverse polarity. In accordance withthis model, the interval between the maximum and minimum points of thelarge peaks is longer for the silver strip 170 with a length fraction of0.3, shown in FIG. 7b.

An interval of minor waves separates the large ones as shown in FIG. 7a.This interval could be the electrical signature of uncoated aluminum Thefluctuation could reflect microstructural variation in the aluminumsurface that also gives rise to heterogeneity in the work function. Themicrostructural variation could be linked to the machining history ofthe shaft 100.

The amplitudes of both the maximum and minimum peaks of the major waveis influenced strongly by the rotational frequency of the shaft 100 andthe capacitance spacing. As an example, a quantitative analysis of themaximum peak measured for a silver strip with a length fraction of 0.013is presented. FIG. 8 shows that the magnitude of the maximum peakdeclines non-linearly from 2.8 to 0.9 V with probe distance. It shouldbe noted that the curves in FIG. 8 have identical shape; however, theyshift to higher voltages as the rotational frequency increases from 10to 25 Hz.

A mathematical equation for each curve in FIG. 8 can be derived bylinearization. This is done by plotting the natural logarithm of themaximum voltage (V_(max)) against that of the distance, and thencalculating the slope (s) and y-intercept (y) through linear regression.FIG. 9 reveals that the fit (r²) of the linearized curves ranges from0.99 to 1.00. Such excellent r² values confirms the validity of thecurve fitting technique being applied. Rearranging the linear equation1n(V_(max))=[s × 1n(d)]+y  (10)yields an asymptotic expression for V_(max)V_(max)=c/d³  (11)where c=e^(y). Equation 11 takes into account the negative slopeindicated by the linearized plots in FIG. 9. Table 1 shows the values ofc and s for each rotational frequency.

TABLE 1 Frequency (Hz) c s 10 0.874 0.6 15 2.230 0.8 20 1.565 0.9 252.040 0.8

The empirical equation for V_(max) conforms with the predicted model forthe induced current (equation 7). Both equations are asymptotic;however, the experimental value of s in equation 9 range from 0.6 to0.9. Except for f=10 Hz, these values are slightly below 1, which is thepredicted value. It should be noted that the probe signal is acquiredthrough a current-to-voltage conversion circuit with a gain factor of3.9×10⁹ V/amp. Taking this and equation 7 into account, it is proposedthat the numerator, c, in the empirical equation, represents a productof the induced current, conversion factor, dielectric constants anddV/dt. Among these parameters, dV/dt which increases linearly with therotational frequency, shown in FIG. 4, is variable.

FIG. 10 shows that, at a constant d, the magnitude of the maximum peakincreases linearly with the rotational frequency and the slope for eachline increases with decreasing spacing distance.

Therefore, the applicability of the non-vibrating capacitance probe fordetecting surface variation in the work function has been presented.This variation is reflected by the nature of the current induced by thechanging contact potential difference between the reference electrodeand the surface in question. The magnitude of the induced current whichindicates the sensitivity of the probe, decreases asymptotically withdistance between the probe and sample, and increases linearly with therate of CPD change. These results are consistent with the theoreticalmodel.

While the invention has been disclosed in its preferred forms, it willbe apparent to those skilled in the art that many modifications,additions, and deletions can be made therein without departing from thespirit and scope of the invention and its equivalents as set forth inthe following claims.

1. An apparatus for monitoring surface variations on a component, saidapparatus comprising: (a) a non-vibrating capacitance probe; (b) meansfor positioning said non-vibrating capacitance probe in proximity to thecomponent; and (c) means for measuring the contact potential differencearising from relative motion between the component and saidnon-vibrating capacitance probe and changes in the contact potentialdifference being characteristic of correlated surface variations of thecomponent.
 2. An apparatus according to claim 1, further comprising ameans for measuring the relative motion between the component and saidnon-vibrating capacitance probe.
 3. An apparatus according to claim 2,further comprising means for regulating the relative motion between thecomponent and said non-vibrating capacitance probe.
 4. An apparatusaccording to claim 1, further comprising means for measuring the spatialdistance between the component and said non-vibrating capacitance probe.5. An apparatus according to claim 1, further comprising a means forsupporting the component further including means for scanning whichprovides spatially continuous scanning of the probe relative to thecomponent.
 6. An apparatus according to claim 5, further comprising ameans for supporting the component, wherein said means for positioningsaid non-vibrating capacitance probe in proximity to the component isfixed relative to said means for supporting the component.
 7. Anapparatus according to claim 1, wherein said surface variations iscomprise surface wear.
 8. A process for monitoring surface variations ona component, comprising the following steps: (a) imparting relativemotion between the component and a non-vibrating capacitance probe; (b)monitoring the relative motion between the component and thenon-vibrating capacitance probe; and (c) monitoring the contactpotential difference between the component and the non-vibratingcapacitance probe with changes in the contact potential differencecharacteristic of correlated surface variations of the component.
 9. Aprocess according to claim 8, further comprising the step of monitoringthe distance between the said test surface and the non-vibratingcapacitance probe.
 10. A process according to claim 9, wherein thesurface variation is surface wear.
 11. A non-contact detector formeasuring a property of a sample comprising, a non-vibrating sensorbeing in electrical communication with a sample, the sample and thenon-vibrating sensor having different work functions and being separatedfrom one another by a characteristic distance, and a measurement devicefor measuring a current directly related to a temporal variation of acontact potential difference between the sample and the sensor, therebymeasuring a property of the sample.
 12. The non-contact detector ofclaim 11 , the sensor being a non-vibrating sensor which is structurallymoved relative to the sample.
 13. A non-contact detector for measuringat least one of chemical properties and tribological wear of a componentcomprising: (a) a non-vibrating sensor having a sensor work function,the non-vibrating sensor being in proximity to the component at aselected distance from the component and the non-vibrating sensorscanned relative to the component, and the component having a componentwork function; and (b) a measurement device for measuring a temporalvariation in a property relatable to the component work function and thetemporal variations in a property selected from the group of acorrelated change in surface composition of the component, change in thetribological wear of the component and spatial variations of thecomponent.
 14. The non-contact detector of claim 13 , the sensor workfunction being different than the component work function.
 15. Thenon-contact detector of claim 14 , the measurement device for measuringthe temporal variation in the component work function wherein theproperty is determined by measuring an induced current which is relatedto a temporal change in contact potential difference between thecomponent and the sensor.
 16. An apparatus for monitoring surfacechanges on a component, said apparatus comprising: (a) a non-vibratingcapacitance probe; (b) a placement device for positioning thenon-vibrating capacitance probe in proximity to the component and asystem for scanning the probe relative to the component; and (c) a firstmeasurement device for measuring a property which is relatable to thecontact potential difference between the component and the non-vibratingcapacitance probe and the property relatable to the contact potentialdifference arising from at least one of a compositional surface changeof the component, spatial variation and tribological wear.
 17. Anapparatus according to claim 16, further comprising a second measurementdevice for measuring the relative motion between the component and thenon-vibrating capacitance probe.
 18. An apparatus according to claim 17,further comprising a regulator capable of regulating the relative motionbetween the component and the non-vibrating capacitance probe.
 19. Anapparatus according to claim 16, further comprising a third measurementdevice for measuring a nearest spatial distance between the componentand the non-vibrating capacitance probe.
 20. An apparatus according toclaim 16, further comprising a support for supporting the component. 21.An apparatus according to claim 20, wherein the placement device forpositioning the non-vibrating capacitance probe in proximity to thecomponent is fixed relative to the support.
 22. A capacitance probe formeasuring at least one property of a sample, comprising: (a) a referenceelectrode and a sample forming at least part of an electrical circuit,the reference electrode disposed adjacent the sample and having acharacteristic closest separation distance between the sample and thereference electrode, the reference electrode maintained substantiallyfixed during measurement of the at least one property, and the sampleand the reference electrode forming a capacitor element of theelectrical circuit; (b) a voltage source coupled to the referenceelectrode and being part of the electrical circuit; and (c) a device formeasuring current induced by activating the voltage source in theelectrical circuit, the measured current arising from a temporal changein the contact potential difference between the reference electrode andthe sample with the temporal change associated with a change of at leastone of a compositional change of the sample, tribological wear of thesample and a change of distance between the reference electrode and thesample.
 23. The capacitance probe of claim 22, the reference electrodebeing a non-vibrating reference electrode.
 24. A non-contact detectorfor measuring at least one of tribological wear and chemical changes ofa sample comprising, a non-vibrating sensor being in electricalcommunication with a sample, the sample and the sensor having differentwork functions and being separated from one another by a selecteddistance of closest approach and a measurement device for measuring acurrent related to a time varying change in the selected distance ofclosest approach between the sample and the sensor, thereby measuringthe at least one of tribological wear and chemical changes of thesample.
 25. The non-contact detector of claim 24 wherein thetribological wear comprises mechanical defect surface variations of thesample.
 26. A method of sensing at least one of chemical properties andtribological wear of a sample comprising the steps of: positioning anon-vibrating sensor in proximity to the sample, the sensor beingseparated by a selected distance from the sample; scanning thenon-vibrating sensor relative to the sample; and measuring a currentrelated to a contact potential difference between the sample and thesensor and analyzing the current to determine at least one of thechemical properties and tribological wear of the sample.
 27. A method ofsensing at least one of chemical properties and tribological wear of asample comprising the steps of: locating a non-vibrating sensor having asensor work function in proximity to the sample having a sample workfunction, the sensor being separated by a selected distance from thesample; scanning the non-vibrating sensor relative to the sample;measuring an induced current between the sample and the sensor; anddetermining at least one of chemical properties and tribological wear ofthe sample by relating the induced current to at least one of (i) adifference between the sensor work function and the sample work functionand (ii) a variation in the selected distance from the sample.
 28. Anon-contact detector for measuring a work function characteristic of amaterial at the surface of a component comprising: a sensor having asensor work function, the sensor disposed in proximity to the componentat a selected distance from the component, and the component having acomponent work function; a mechanism to drive at least one of thecomponent and the sensor laterally relative to one another; and ameasurement device for measuring a temporal variation in work functionof difference between the sensor and the component over a spatial rangealong the component, arising from the sensor moving laterally relativeto the component to determine variation of the component work functionover the spatial range of the material and in turn properties of thesurface of the component.
 29. A non-contact detector for determiningdifferences of contact potential difference at locations along thesurface of a component having a component work function, comprising: (a)a non-vibrating sensor having a sensor work function and when thenon-vibrating sensor is disposed in proximity to and scanned relative tothe component, a surface charge is detected as a result of the temporalchange of the work function of the component; and (b) a measurementsystem which uses the surface charge sensed by the non-vibrating sensorto determine a contact potential difference for the component as thesensor is scanned relative to the surface of the component, the contactpotential difference changes being characteristic of changes ofcomposition of the material at the surface of the component along aspatial line of the component.
 30. The non-contact detector as definedin claim 29 wherein the changes in the contact potential differencecomprise microstructural variation of the component surface.
 31. Thenon-contact detector as defined in claim 30 wherein the measurementsystem provides a quantitative analysis result for the surface of thecomponent.
 32. A non-contact detector for performing quantitativeanalysis of the surface of a component, comprising: a sensor having asensor work function and the sensor when disposed in proximity to thecomponent and scanned relative to the component senses a temporal changeof the work function when passing from one material to another materialof the component along a spatial dimension of the component; and asystem for analyzing the temporal change of the work function tocharacterize at least one of composition and quantitative measure ofdimensional changes at the surface of the component along the spatialdimension of the component.
 33. A method of determining differences ofcontact potential difference for a component, comprising the steps of:positioning a sensor near a component surface, the sensor having asensor work function; scanning the sensor laterally relative to thecomponent along a line, the scanning generating a surface charge when atemporal change of the work function occurs along the line; andmeasuring the surface charge over the line and characterizing at leastone of composition and wear of the surface of the component.
 34. Amethod of monitoring surface variations on a component, comprising thesteps of: positioning a non-vibrating capacitance probe near a componentbeing monitored; scanning the non-vibrating capacitance probe relativeto the component; and measuring along a line the contact potentialdifference between the component and said non-vibrating capacitanceprobe, with measured changes at points along the line of the contactpotential difference being characteristic of correlated surfacevariations of the component.
 35. A method according to claim 34, whereinthe scanning step provides spatially continuous scanning of the proberelative to the component, thereby allowing mapping of the surfacevariations of the component.
 36. A process for monitoring surfacevariations on a component, comprising the following steps: (a)determining a contact potential difference for a component by impartingrelative lateral motion between the component and a capacitance probe;(b) monitoring the relative lateral motion between the component and thecapacitance probe to identify location of surface variations on thecomponent; and (c) monitoring the contact potential difference betweenthe component and the capacitance probe with changes in the contactpotential difference characteristic of surface variations of thecomponent which are then correlated to the location on the component.37. The method as defined in claim 36 wherein the relative lateralmotion maps a line of points on the component characteristic of thecorrelated surface variations of the component.